0000365: Should store TITLE, META DESCRIPTION and KEYWORDS in history and show their changes in /revisions and /diff

Description

Now wakco ignores history of changes in TITLE, META DESCRIPTION and KEYWORDS and doesn't show their changes in /revisions and /diff. That's not right - for SEO purposes one needs to track the date of changing TITLE, META DESCRIPTION and KEYWORDS and their values.
To make this work for us in /revisions and /diff we should:
1. Add fields TITLE, META DESCRIPTION and KEYWORDS to wacko_revision
2. Add creation of a new revision when changing TITLE, META DESCRIPTION and KEYWORDS in handler /settings
3. Add showing changes in TITLE, META DESCRIPTION and KEYWORDS in handler /diff